Today we will cover a guide on how to weld cast iron. But before we delve into this topic, it is important you understand what the material is that needs to be welded.
What is Cast Iron?
There are many types of metals used widely for development and other specialized purposes. The combination of carbon with iron results in the metal known as cast iron.
Cast iron has been used for centuries due to its widespread benefits such as durability and resistance to general wear down over time. The popularity of cast iron has not died down amid the rise of other metals.
So, without any further delay, let’s begin our guide including advice on welding cast iron whether you are a beginner or an expert.
Can You Weld Cast Iron?
Now that you know what a cast iron is, one of the first things that we need to clarify next is whether or not cast iron be welded.
Welding cast iron is possible and it can be welded however, it is not an easy task!
As we have mentioned a minute ago, cast iron is used widely however, in the many years of its usage it has built up a reputation for being one of the most stubborn metals to be welded.
While it is durable and highly resistant to wear, this becomes an obstacle when welding. This is because of the higher levels of carbon in its composition ranging between 2% to 4%. The carbon content makes it hard and brittle during the welding process due to carbon migration.
As a result, cast iron is sensitive in terms of the brittleness for welding. But the different grades of cast iron will have differing degrees of brittleness. So, the potential cracks that may result when welding cast iron is not the same for every piece of cast iron.
Is it possible to weld cast iron?
It is very difficult to weld cast iron but certainly not impossible!
Why Welding Cast Iron Is Difficult?
We have already learned that welding cast iron is possible but why is it so difficult?
The difficulty of cast iron is the welding process itself and the weldability of cast iron varies based on the type and grade of cast iron. Grey cast iron is the most common type of cast iron.
The most frequent use of welding cast iron involves the repairing of castings. The repairs may be needed due to defects of mistakes. For example, when a hole is drilled in the wrong place of a cast iron.
Also, when cast iron parts are broken for some reason and need repairing. Since cast iron has the tendency of being brittle, it is unusual for them to break.
The following are some of the reasons that explain the difficulty of welding cast iron:
- Cast iron has a very high carbon content between 2% to 4%. This is more than 10 times that of other steels.
- Because of the high levels of carbon, flakes of graphite form when welding.
- When making castings the cooling process is extremely important as it makes the difference between cracked and crack free castings.
- During and after welding, cast iron needs to cool slowly for a lengthy time.
- It is important to keep in mind the milestone of 1450 degrees F because this is the temperature when cracking can occur. So, when welding be mindful that it is not kept at this temperature for too long.
How to Weld Cast Iron
We know that welding cast iron is difficult but not impossible and now for the main course – how to weld cast iron!
As a beginner, or as an expert, there are certain challenges that must be overcome for effective in-house welding of cast iron. The effectiveness will show in terms of the time and money saved in the process.
We have learned already that cast-iron welding failure can result in cracking or irreparable damages. It is always suggested to work with experienced welders if you are unsure of doing it yourself. The following are the five (5) major components of welding cast iron:
- Identify the type of metal
- Ensure thorough cleaning
- Pre-heat is crucial
- Choose welding technique
PRO TIP: Even before you begin thinking about welding cast iron, you must identify, clean, preheat and select a welding technique.
STEP 1: IDENTIFY THE CAST IRON
We have already discussed how there are different types of cast irons and the carbon content of them will vary which will impact your welding experience.
So, it is crucial that you identify the exact type of cast iron. In comparison to other metals, cast iron has different characteristics.
As a rule of thumb, you must know that cast irons will not simply stretch when heated, they will crack – that’s what you want to avoid!
The following are common types of cast irons and you will see how some are easier than others to weld:
Grey Cast Iron
The most common type of cast iron is known as grey cast iron.
We already discussed how the carbon in grey cast iron can turn into graphite flakes while welding. It has a better weldability compared to white cast iron but it has a challenge as well.
The graphite flakes of the grey cast iron can cause the metal to become highly brittle during welding.
White Cast Iron
In the case of white cast iron, the carbon doesn’t fall off as graphite flakes rather it holds on as carbide.
As a result, the cementite crystalline microstructure which is formed is extremely hard and brittle making it virtually impossible to weld.
So, if you identify the metal is white cast iron, then don’t bother trying to weld it.
STEP 2: CLEAN THE CASTIRON
Regardless of the specific type of cast iron you have identified, all castings must go through a proper, thorough cleaning before any welding work is done. Whether it is debris or other surface materials, they must all be removed.
At the very least, the welding area must be completely cleaned. So, if there is grease, paint, oil or anything else that is not the cast iron itself – it must be removed.
You can even apply a little bit of heat in the welding area to remove trapped gases to get to the base metal. You can also hold pass the weld quickly to see if there are any foreign deposits that need to be cleaned.
Once you have found the cast iron to be fully ready, then you can move on to the next crucial step of preheating the cast iron.
STEP 3: PRE-HEAT CAST IRON
By now, without a shadow of doubt, we know that cast irons are sensitive to cracking when welded. So, special care must be given in terms of the exposure to high temperatures. Which is why, preheating is an important aspect before the welding process.
The following are the types of heat control that can be applied on cast iron to prepare it for welding:
- Low heat input
- Slow cooling
Heat control will allow for proper thermal expansion for cast iron without cracking. One of the above methods must be applied to ensured for success in welding cast iron.
In general, preheating is preferred and highly recommended when welding cast iron. So, in the debate of whether to preheat or not, definitely preheat!
Another method of heat control is low heat input on cast iron. When you apply low heat then the surrounding metal is still cool allowing restricted expansion. If you are unable to ensure that enough preheating is not possible, then the next best thing to do is weld in a lower temperature.
You can also weld cast iron by the heat control method of keeping it cool or slow cooling. It is important to recognize the difference between cool and not cold. Do not alternate between preheating and cooling, stick with one or the other.
The cooling rate is important because just as extreme heat can cause it to crack, rapid cooling can also cause contraction, followed by brittle casting that will easily crack.
STEP 4: CHOOSE CAST IRON WELDING TECHNIQUE
Once you have identified, cleaned and preheated the cast iron, it is time for you to choose an appropriate welding technique.
It is important to note that the welding technique is not simply a matter of preference, rather it is based on the exact suitability of the cast iron being welded.
There are 3 main welding techniques that we will briefly discuss when welding cast iron:
Stick welding is a common welding technique which is more commonly known as shielded metal arc welding or MMA. It uses electrodes covered with a flux for welding.
There are different types of electrodes that can be used such as cast iron covered, copper allow and nickel alloy electrodes. The type of electrode is selected depending on the type of application and the color match required for the job.
Nickel alloy electrodes are the most popular choice when welding cast iron. It is stronger and has a lower potential for thermal expansion, as a result, there is reduced chance of cracking. There is also an added concern of the amount of machining required after welding.
Oxy Acetylene Welding
Oxy acetylene welding also uses electrodes and an oxy acetylene torch provides the energy needed for the welding.
There is a certain level of precaution and care needed when using this technique, so that the cast iron is not oxidized during welding. If so, it can cause the formation of white iron in the weld.
Braze welding is also a very common welding method for cast iron. It is used mainly for joining cast iron parts. In this technique, a welding rod acts as a filler for the cast iron and it fixes itself on the surface.
Regardless, of which method you use, the cleanliness of the cast iron surface is absolutely crucial. Especially for braze welding technique, which relies on the cast iron surface for attaching the welding rod.
So, when choosing a welding technique, you should be careful and choose based on the specific type of the cast iron you are welding.
STEP 5: FINISHING THE WELDING
Now for the big finish for your welding job, it is essential that you look for the most dreaded outcome – cracking!
As we have discussed throughout, the ultimate danger of welding cast iron is cracking. Which is why, we have gone through this brief yet essential step by step process to ensure that you get the perfect weld.
Now, what happens when you have some cracking and you need to finish the job?
One option for you to seal cracks, now these are very tiny cracks that must be polished for the perfect finish. These are tiny ones that appear even after you ensure all the safe procedures and best practices. So, don’t be alarmed.
Simply, use a form of sealant or sealing compound to ensure that there is no leaking based on the environment of the cast iron.
Another method is required when cracks are slightly larger.
To do this, you have to drill on beveled areas using steel studs. You must screw the steel studs into the holes and leave it unscrewed a certain amount.
As a result, you will weld the studs and cover the entire surface using the weld deposits. This way both sides can be welded together.
Remember, the final step of welding cast iron process is cooling – so you must make sure that you expose the cast iron to a slow cooling process.
Welding Cast Iron: Tips, Tricks & Best Practices
The chances of cracking can be massively decreased by applying these steps and for more, we will provide a list of the best practices which will serve as tips and tricks to get it done right, every time.
Here are the best practices to follow for best results:
- Do not skip heat control – always preheat the cast iron to control the expansion and contraction using one of the 3 mentioned techniques: preheating, low heating or slow cooling.
- Work on a warm environment as in the cast iron itself and allow plenty of time such as for slow cooling to end the process and make sure to never rush the process.
- Work with you welding rod in a vertical position for most control.
- Weld in batches – for example, let one cast iron cool while you move on to another for welding or preheating, etc. this will make sure you are not rushing through any of the steps.
- Be sure to clean up after every weld so that you do not transfer debris from one piece to the next.
- Consider the sealing and studding methods for getting the perfect finish for your welding, this will also stop the expansion of tiny existing cracks.
- Be sure to repeat the cleaning step throughout the process such as regularly removing any noticeable debris such as rust, oil grease and any other contaminants.
- Take as much time as possible when welding cast iron.
That brings us to the end of this guide on welding cast iron as a beginner.
We know for sure that it is NOT impossible to cast iron however, it is quite difficult. So, it is advised that both beginners and experts use a proper process when welding cast iron.